Mass Transport in the Stratosphere Examined Using the Transformed Eulerian-Mean Residual Circulation.
The transformed Eulerian-mean (TEM) residual circulation is used to study the zonally averaged transport of mass in the stratosphere. Estimates of the residual circulation are made two ways. The first consists of solving for the residual circulation stream function using the TEM zonal momentum and continuity equations using estimates of the large scale momentum forcing based on several years of data (downward control method). The second method solves for the residual circulation using the TEM thermodynamic energy and continuity equations using heating rates computed with a radiative transfer model (radiatively derived). An error analysis of the downward control method shows that the method works well under steady state conditions and when all the forcings for the zonal momentum equation are known. It appears possible to construct reasonable estimates of net tropical upward mass flux without knowledge of the sub-grid scale forces. However, such forces are important for determining residual velocities at middle and high latitudes. Estimates of the residual circulation done using the downward control method show a larger tropical upward mass flux across the 100 mb surface during DJF than during JJA. Such an annual cycle in mass flux is also present in the radiatively derived residual circulation. Via a "downward control" interpretation, this can be considered to be the result of a greater extratropical zonal momentum force in the DJF winter hemisphere. Radiatively derived residual velocities are used to investigate the relationship between the annual temperature and upward mass flux oscillations in the tropical lower stratosphere. Temperature tendency in the tropical lower stratosphere is found to follow vertical advection of temperature as opposed to local radiative heating. The annual tropical lower stratospheric temperature oscillation appears to be a response to a corresponding oscillation in vertical velocities. It is suggested that the annual oscillation in tropical vertical velocities in the lower stratosphere is the result of a greater extratropical zonal momentum force in the DJF winter hemisphere. Wave activity in the Northern Hemisphere appears to be controlling the seasonal cycle in mass flux across a pressure surface in the lower stratosphere both in the Northern Hemisphere and in the tropics. These hemispheric asymmetries exist presumably as the result of hemispheric asymmetries in the earth's surface.
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- Physics: Atmospheric Science; Geophysics